The triple oxygen isotope composition of sulphate minerals has been used to constrain the evolution of Earth's surface environment (e.g., pO2, pCO2 and gross primary productivity) throughout the Proterozoic Eon. This approach presumes the incorporation of atmospheric O2 atoms into riverine sulphate via the oxidative weathering of pyrite. However, this is not borne out in recent geological or modern sulphate records, where an atmospheric signal is imperceptible and where terrestrial pyrite weathering occurs predominantly in bedrock fractures that are physically more removed from atmospheric O2. To better define the transition from a Proterozoic to a modern-like weathering regime, here we present new measurements from twelve marine evaporite basins spanning the Phanerozoic. These data display a step-like transition in the triple oxygen isotope composition of evaporite sulphate during the mid-Paleozoic (420 to 387.7 million years ago). We propose that the evolution of early root systems deepened the locus of pyrite oxidation and reduced the incorporation of O2 into sulphate. Further, the early Devonian proliferation of land plants increased terrestrial organic carbon burial, releasing free oxygen that fueled increased redox recycling of soil-bound iron and resulted in the final rise in pO2 to modern-like levels.

Accurately reconstructing the temperature of the ocean through time carries implications for Earth's climate and early habitability. Attempts to build these reconstructions using oxygen isotope records have led to three end-member interpretations. Namely, that the observed enrichment over time in 18O relative to 16O in ancient chemical sediments reflects a change in sea-surface temperatures (SST), a change in the 18O composition of the contemporaneous water, or that the primary signal has been subsequently overprinted. These questions become most salient in the Archean, where estimates of the isotopic composition of the ocean span ∼20‰, with a correspondingly wide range in estimated SSTs. Here, we introduce barite (BaSO4) as a robust new proxy for the oxygen isotope composition of the Archean ocean. We compile new and existing triple oxygen isotope and sulfur isotope data from the Fig Tree Group barite deposits in the Barberton Greenstone Belt, South Africa, with the goal of identifying the primary sources of sulfate to the Archean ocean. Using a simple Monte Carlo approach, we then constrain the possible isotopic composition of contemporaneous seawater. Our results suggest that microbial sulfur cycling played a limited role in setting the isotopic composition of Archean seawater sulfate. Additionally, it is likely that a significant flux of sulfate to the marine reservoir was atmospherically derived and carried a significant positive triple oxygen isotope signal. Importantly, our results support an Archean ocean with a somewhat enriched oxygen isotope composition (∼0-5‰), with the exact composition dependent on the relative contribution from each sulfate production pathway. This result points either to the decreased significance of low-temperature weathering and/or to elevated SSTs in the early Archean.

The triple oxygen isotope composition of seawater sulfate, as recorded in marine sulfate evaporites and barites, is commonly used to interpret past changes in atmospheric pO(2)/pCO(2) and gross primary production (GPP). In practice, the most-negative measured triple oxygen isotope value (Delta'O-17) of sulfate from a marine evaporite deposit is thought to most closely represent contemporaneous seawater sulfate and is used to calculate atmospheric composition. However, a range of triple oxygen isotope compositions are typically measured within a single marine evaporite basin. Here, we characterize in detail the variability in the triple oxygen isotope composition of the sulfate in gypsum sampled from three Messinian (5-6 Ma) marine evaporite sub-basins from the Western Mediterranean Basin. Evaporite sulfate is offset from contemporaneous seawater sulfate and reflects mixing between two end-member sulfate populations: the original seawater sulfate and sulfate that has been isotopically reset after basin restriction. The combined Delta'O-17 and delta O-18 compositions of sulfate within a stratigraphic context offer the opportunity to better constrain the degree to which marine sulfate evaporites preserve the original isotopic composition of open ocean seawater sulfate. This study encompasses an exploration of mass-dependent fractionation, isotope equilibrium with water, and various scenarios of mixing. Our results calibrate the utility of marine sulfate evaporites in constraining the contemporaneous, open ocean triple oxygen isotope composition of seawater sulfate. (C) 2021 Elsevier B.V. All rights reserved.

SignificancePyrite is oxidized during weathering to form dissolved sulfate that is carried to the ocean by rivers. This process is thought to incorporate atmosphericO2-derived oxygen; geologically preserved sulfate has thus been proposed to directly trace pastO2isotope compositions. However, this mechanism has not been thoroughly tested in modern weathering environments. We show that dissolved sulfate in Himalayan rivers is predominantly derived from pyrite, yet its oxygen isotope compositions preclude directO2incorporation. Rather, alternative oxygen sources (e.g., reactive oxygen species) may be incorporated during oxidation, prompting reconsideration of the pyrite oxidation mechanism and the interpretation of geologically preserved sulfate as a directO2tracer.

This mixed methods study examined the vocabulary knowledge and instructional practices of four grade 1 teachers in order to better understand how prepared teachers are to implement the First Grade English Language Arts Common Core State Standards (CCSS). Data collected included an audiotaped semi-structured interview, a teacher survey, video-recorded class vocabulary instruction, researcher field notes, and artifacts of class activities. Findings indicate that teaching experience is not a factor in the level of vocabulary instruction teachers provide. For example, veteran teachers in the analysis rely on traditional spelling tests and basal based vocabulary instruction to teach vocabulary, while the less experienced teachers utilize trade books to guide their instruction. However, veteran teachers provide vocabulary instruction daily, while less experienced teachers did not. Findings suggest that all teachers would benefit from professional learning opportunities related to vocabulary instruction and researchers are encouraged to investigate vocabulary instruction and its alignment to the CCSS with larger sample sizes in other districts and across all grade levels.